Undefined and NaN

Two special values you will meet often.

undefined

A variable with no assigned value is undefined:

js
let x;
console.log(x); // undefined

Also returned by missing object properties.

null

null is an intentional empty value. It means "nothing here on purpose":

js
let x = null;

undefined vs null

• undefined: no value was ever set.
• null: the value was explicitly set to "nothing".

NaN

NaN means "Not a Number". It results from invalid math:

js
"abc" * 2; // NaN

### Number("abc"); // NaN

Checking NaN

NaN is the only value not equal to itself. Use Number.isNaN():

js

### NaN === NaN;          // false (surprise!)

### Number.isNaN(NaN);    // true

### Number.isNaN("abc");  // false ("abc" is not NaN)

TL;DR

• undefined means no value was assigned.
• null is an intentional empty value.
• NaN results from invalid math.
• Use Number.isNaN() to check NaN.